Microfluidics generally refers to microfabricated devices, which are used for pumping, sampling, mixing, analyzing and dosing liquids. Prominent features thereof originate from the peculiar behavior that liquids exhibit at the micrometer length scale. Flow of liquids in microfluidics is typically laminar. Volumes well below one nanoliter can be reached by fabricating structures with lateral dimensions in the micrometer range. Reactions that are limited at large scales (by diffusion of reactants) can be accelerated. Finally, parallel streams of liquids can possibly be accurately and reproducibly controlled, allowing for chemical reactions and gradients to be made at liquid/liquid and liquid/solid interfaces. Microfluidics are accordingly used for various applications in life sciences. Microfluidic devices are commonly called microfluidic chips.
Microfluidic-based bioassays require passing a liquid sample inside a microfluidic flow path. The flow conditions (volume passing and flow rate) are important as they impact the outcome of the assay. While several methods and devices for flowing liquids inside microfluidic flow paths have been developed, these methods either lack flexibility or operate with a limited type of samples and flow conditions.
For instance, a number of microfluidic devices enabling capillary-driven flows (CDFs) have been developed. In a related field, some microfluidic devices enabling electroosmotic (EO) flows have been developed. For example, an EO microfluidic device may comprise a microchannel defined between glass walls and comprising opposite electroosmotic electrodes, provided at two ends of the microchannel, see e.g., “A Planar Electroosmotic Micropump” Chen, C and Santiago, J. G., J. Microelectromechanical Systems, 2002, 11, 672-683. In such a device, glass acquires a negative surface charge upon contact with an aqueous solution (resulting in electric double layer). In the EO flow, mobile ions in the diffuse counter-ion layer of the electric double layer are driven by an externally applied electrical field. These moving ions drag along bulk liquid through viscous force interaction.
Besides, the fabrication of microfluidic chips using semiconductor wafers such as Si wafers seems attractive: one may expect to benefit from a range of existing processes, as continuously developed in the past decades for integrated circuits, to obtain accurate microfluidic structures. However, contrary to what is done in semiconductor wafer processing, microfluidics generally have deep structures, i.e., around a few micrometer, up to 20 micrometers or even deeper. In many cases, 5 micrometers is already considered as a small depth in microfluidic applications because such a small depth can generate a large hydraulic resistance on a liquid and can block or become clogged with microbeads and particles, such a small depth can also be incompatible with samples containing cells. As a result, existing semiconductor wafer processes are challenged by, if not incompatible with the requirements needed for microfluidic chip fabrication both in terms of manufacturing processes and cost of fabrication.
In many microfluidic applications, metallic patterns are desirable, e.g., for performing electrochemistry and electro-based detection of analytes, for electrical separation of analytes, or for moving liquids using electro-osmotic flow (EOF), performing dielectrophoresis (DEP), etc. Thick resists (e.g. SU-8) are sometimes used to directly form sidewalls of deep structures.